Erasing memory cells

ABSTRACT

Methods include applying a first voltage to channel regions of a plurality of memory cells; applying a lower second voltage to each access line of a plurality of access lines coupled to the memory cells other than a first set of access lines; applying a lower third voltage to the first set of access lines while applying the first voltage and the second voltage; determining a desired voltage level of the third voltage for a subsequent set of access lines; and applying the third voltage to the subsequent set of access lines while applying the first voltage and while applying the second voltage to each access line of the plurality of access lines other than the subsequent set of access lines. Methods further include methods of determining the desired voltage level for the third voltage for each set of access lines.

TECHNICAL FIELD

The present disclosure relates generally to memory and, in particular,in one or more embodiments, the present disclosure relates to erasingmemory cells.

BACKGROUND

Memory devices are typically provided as internal, semiconductor,integrated circuit devices in computers or other electronic devices.There are many different types of memory including random-access memory(RAM), read only memory (ROM), dynamic random access memory (DRAM),synchronous dynamic random access memory (SDRAM), and flash memory.

Flash memory has developed into a popular source of non-volatile memoryfor a wide range of electronic applications. Flash memory typically usea one-transistor memory cell that allows for high memory densities, highreliability, and low power consumption. Changes in threshold voltage(Vt) of the memory cells, through programming (which is often referredto as writing) of charge storage structures (e.g., floating gates orcharge traps) or other physical phenomena (e.g., phase change orpolarization), determine the data state (e.g., data value) of eachmemory cell. Common uses for flash memory and other non-volatile memoryinclude personal computers, personal digital assistants (PDAs), digitalcameras, digital media players, digital recorders, games, appliances,vehicles, wireless devices, mobile telephones, and removable memorymodules, and the uses for non-volatile memory continue to expand.

A NAND flash memory is a common type of flash memory device, so calledfor the logical form in which the basic memory cell configuration isarranged. Typically, the array of memory cells for NAND flash memory isarranged such that the control gate of each memory cell of a row of thearray is connected together to form an access line, such as a word line.Columns of the array include strings (often termed NAND strings) ofmemory cells connected together in series between a pair of selectgates, e.g., a source select transistor and a drain select transistor.Each source select transistor may be connected to a source, while eachdrain select transistor may be connected to a data line, such as columnbit line. Variations using more than one select gate between a string ofmemory cells and the source, and/or between the string of memory cellsand the data line, are known.

Memory cells are typically erased before they are programmed to adesired data state. For example, memory cells of a particular block ofmemory cells may first be erased and then selectively programmed. For aNAND array, a block of memory cells is typically erased by grounding allof the access lines (e.g., word lines) in the block and applying anerase voltage to the channel regions of the memory cells (e.g., throughdata lines and source connections) in order to remove charges that mightbe stored on data-storage structures (e.g., floating gates or chargetraps) of the block of memory cells. Typical erase voltages might be onthe order of 25V before completion of an erase operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a memory in communication with aprocessor as part of an electronic system, according to an embodiment.

FIGS. 2A-2B are schematics of portions of an array of memory cells ascould be used in a memory of the type described with reference to FIG.1.

FIG. 3 is a conceptual view of the organization of a die containing anapparatus in accordance with an embodiment having an array of memorycells.

FIG. 4 is a top view of a semiconductor wafer containing a number ofdies containing an apparatus in accordance with an embodiment having anarray of memory cells.

FIGS. 5A-5D depict visualizations of erase threshold voltagedistributions for individual access lines and resulting erase thresholdvoltage distributions.

FIG. 6A is a cross-sectional view of a portion of a block of memorycells as might be used with various embodiments.

FIG. 6B is a simplified cross-sectional view of a portion of a block ofmemory cells as might be used with various embodiments.

FIG. 7 depicts a visualization of erase threshold voltage distributionsfor individual access lines in accordance with an embodiment.

FIG. 8 is a flowchart of a method of operating a memory in accordancewith an embodiment.

FIG. 9 is a flowchart of a method of operating a memory in accordancewith an embodiment.

FIG. 10 depicts waveforms of access lines and channel regions for amethod of operating a memory in accordance with an embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown, byway of illustration, specific embodiments. In the drawings, likereference numerals describe substantially similar components throughoutthe several views. Other embodiments may be utilized and structural,logical and electrical changes may be made without departing from thescope of the present disclosure. The following detailed description is,therefore, not to be taken in a limiting sense.

The term “semiconductor” used herein can refer to, for example, a layerof material, a wafer, or a substrate, and includes any basesemiconductor structure. “Semiconductor” is to be understood asincluding silicon-on-sapphire (SOS) technology, silicon-on-insulator(SOI) technology, thin film transistor (TFT) technology, doped andundoped semiconductors, epitaxial layers of a silicon supported by abase semiconductor structure, as well as other semiconductor structureswell known to one skilled in the art. Furthermore, when reference ismade to a semiconductor in the following description, previous processsteps may have been utilized to form regions/junctions in the basesemiconductor structure, and the term semiconductor can include theunderlying layers containing such regions/junctions. The term conductiveas used herein, as well as its various related forms, e.g., conduct,conductively, conducting, conduction, conductivity, etc., refers toelectrically conductive unless otherwise apparent from the context.Similarly, the term connecting as used herein, as well as its variousrelated forms, e.g., connect, connected, connection, etc., refers toelectrically connecting unless otherwise apparent from the context.

FIG. 1 is a simplified block diagram of a first apparatus, in the formof a memory (e.g., memory device) 100, in communication with a secondapparatus, in the form of a processor 130, as part of a third apparatus,in the form of an electronic system, according to an embodiment. Someexamples of electronic systems include personal computers, personaldigital assistants (PDAs), digital cameras, digital media players,digital recorders, games, appliances, vehicles, wireless devices,cellular telephones and the like. The processor 130, e.g., a controllerexternal to the memory device 100, may be a memory controller or otherexternal host device.

Memory device 100 includes an array of memory cells 104 logicallyarranged in rows and columns. Memory cells of a logical row aretypically connected to the same access line (commonly referred to as aword line) while memory cells of a logical column are typicallyselectively connected to the same data line (commonly referred to as abit line). A single access line may be associated with more than onelogical row of memory cells and a single data line may be associatedwith more than one logical column. Memory cells (not shown in FIG. 1) ofat least a portion of array of memory cells 104 are capable of beingprogrammed to one of at least two data states.

A row decode circuitry 108 and a column decode circuitry 110 areprovided to decode address signals. Address signals are received anddecoded to access the array of memory cells 104. Memory device 100 alsoincludes input/output (I/O) control circuitry 112 to manage input ofcommands, addresses and data to the memory device 100 as well as outputof data and status information from the memory device 100. An addressregister 114 is in communication with I/O control circuitry 112 and rowdecode circuitry 108 and column decode circuitry 110 to latch theaddress signals prior to decoding. A command register 124 is incommunication with I/O control circuitry 112 and control logic 116 tolatch incoming commands. A trim register 126 may be in communicationwith the control logic 116 to store trim data. Although depicted as aseparate storage register, trim register 126 may represent a portion ofthe array of memory cells 104.

A controller (e.g., control logic 116 internal to the memory device 100)controls access to the array of memory cells 104 in response to thecommands and generates status information for the external processor130, i.e., control logic 116 is configured to perform access operations(e.g., read operations, program operations and/or erase operations) inaccordance with embodiments described herein. The control logic 116 isin communication with row decode circuitry 108 and column decodecircuitry 110 to control the row decode circuitry 108 and column decodecircuitry 110 in response to the addresses.

Control logic 116 is also in communication with a cache register 118.Cache register 118 latches data, either incoming or outgoing, asdirected by control logic 116 to temporarily store data while the arrayof memory cells 104 is busy writing or reading, respectively, otherdata. During a program operation (e.g., write operation), data is passedfrom the cache register 118 to data register 120 for transfer to thearray of memory cells 104; then new data is latched in the cacheregister 118 from the I/O control circuitry 112. During a readoperation, data is passed from the cache register 118 to the I/O controlcircuitry 112 for output to the external processor 130; then new data ispassed from the data register 120 to the cache register 118. A statusregister 122 is in communication with I/O control circuitry 112 andcontrol logic 116 to latch the status information for output to theprocessor 130.

Memory device 100 receives control signals at control logic 116 fromprocessor 130 over a control link 132. The control signals might includea chip enable CE#, a command latch enable CLE, an address latch enableALE, a write enable WE#, a read enable RE#, and a write protect WP#.Additional or alternative control signals (not shown) may be furtherreceived over control link 132 depending upon the nature of the memorydevice 100. Memory device 100 receives command signals (which representcommands), address signals (which represent addresses), and data signals(which represent data) from processor 130 over a multiplexedinput/output (I/O) bus 134 and outputs data to processor 130 over I/Obus 134.

For example, the commands are received over input/output (I/O) pins[7:0] of I/O bus 134 at I/O control circuitry 112 and are written intocommand register 124. The addresses are received over input/output (I/O)pins [7:0] of I/O bus 134 at I/O control circuitry 112 and are writteninto address register 114. The data are received over input/output (I/O)pins [7:0] for an 8-bit device or input/output (I/O) pins [15:0] for a16-bit device at I/O control circuitry 112 and are written into cacheregister 118. The data are subsequently written into data register 120for programming the array of memory cells 104. For another embodiment,cache register 118 may be omitted, and the data are written directlyinto data register 120. Data are also output over input/output (I/O)pins [7:0] for an 8-bit device or input/output (I/O) pins [15:0] for a16-bit device.

It will be appreciated by those skilled in the art that additionalcircuitry and signals can be provided, and that the memory device 100 ofFIG. 1 has been simplified. It should be recognized that thefunctionality of the various block components described with referenceto FIG. 1 may not necessarily be segregated to distinct components orcomponent portions of an integrated circuit device. For example, asingle component or component portion of an integrated circuit devicecould be adapted to perform the functionality of more than one blockcomponent of FIG. 1. Alternatively, one or more components or componentportions of an integrated circuit device could be combined to performthe functionality of a single block component of FIG. 1.

Additionally, while specific I/O pins are described in accordance withpopular conventions for receipt and output of the various signals, it isnoted that other combinations or numbers of I/O pins may be used in thevarious embodiments.

FIG. 2A is a schematic of a portion of an array of memory cells 200A ascould be used in a memory of the type described with reference to FIG.1, e.g., as a portion of array of memory cells 104. Memory array 200Aincludes access lines, such as word lines 202 ₀ to 202 _(N), and a dataline, such as bit line 204. The word lines 202 may be connected toglobal access lines (e.g., global word lines), not shown in FIG. 2A, ina many-to-one relationship. For some embodiments, memory array 200A maybe formed over a semiconductor that, for example, may be conductivelydoped to have a conductivity type, such as a p-type conductivity, e.g.,to form a p-well, or an n-type conductivity, e.g., to form an n-well.

Memory array 200A might be arranged in rows (each corresponding to aword line 202) and columns (each corresponding to a bit line 204). Eachcolumn may include a string of series-connected memory cells (e.g.,non-volatile memory cells), such as one of NAND strings 206 ₀ to 206_(M). Each NAND string 206 might be connected (e.g., selectivelyconnected) to a common source 216 and might include memory cells 208 ₀to 208 _(N). The memory cells 208 may represent non-volatile memorycells for storage of data. The memory cells 208 of each NAND string 206might be connected in series between a select gate 210 (e.g., afield-effect transistor), such as one of the select gates 210 ₀ to 210_(M) (e.g., that may be source select transistors, commonly referred toas select gate source), and a select gate 212 (e.g., a field-effecttransistor), such as one of the select gates 212 ₀ to 212 _(M) (e.g.,that may be drain select transistors, commonly referred to as selectgate drain). Select gates 210 ₀ to 210 _(M) might be commonly connectedto a select line 214, such as a source select line, and select gates 212₀ to 212 _(M) might be commonly connected to a select line 215, such asa drain select line. Although depicted as traditional field-effecttransistors, the select gates 210 and 212 may utilize a structuresimilar to (e.g., the same as) the memory cells 208. The select gates210 and 212 might represent a plurality of select gates connected inseries, with each select gate in series configured to receive a same orindependent control signal.

A source of each select gate 210 might be connected to common source216. The drain of each select gate 210 might be connected to a memorycell 208 ₀ of the corresponding NAND string 206. For example, the drainof select gate 210 ₀ might be connected to memory cell 208 ₀ of thecorresponding NAND string 206 ₀. Therefore, each select gate 210 mightbe configured to selectively connect a corresponding NAND string 206 tocommon source 216. A control gate of each select gate 210 might beconnected to select line 214.

The drain of each select gate 212 might be connected to the bit line 204for the corresponding NAND string 206. For example, the drain of selectgate 212 ₀ might be connected to the bit line 204 ₀ for thecorresponding NAND string 206 ₀. The source of each select gate 212might be connected to a memory cell 208 _(N) of the corresponding NANDstring 206. For example, the source of select gate 212 ₀ might beconnected to memory cell 208 _(N) of the corresponding NAND string 206₀. Therefore, each select gate 212 might be configured to selectivelyconnect a corresponding NAND string 206 to the common bit line 204. Acontrol gate of each select gate 212 might be connected to select line215.

The memory array in FIG. 2A might be a three-dimensional memory array,e.g., where NAND strings 206 may extend substantially perpendicular to aplane containing the common source 216 and to a plane containing aplurality of bit lines 204 that may be substantially parallel to theplane containing the common source 216.

Typical construction of memory cells 208 includes a data-storagestructure 234 (e.g., a floating gate, charge trap, etc.) that candetermine a data state of the memory cell (e.g., through changes inthreshold voltage), and a control gate 236, as shown in FIG. 2A. Thedata-storage structure 234 may include both conductive and dielectricstructures while the control gate 236 is generally formed of one or moreconductive materials. In some cases, memory cells 208 may further have adefined source/drain (e.g., source) 230 and a defined source/drain(e.g., drain) 232. Memory cells 208 have their control gates 236connected to (and in some cases form) a word line 202.

A column of the memory cells 208 may be a NAND string 206 or a pluralityof NAND strings 206 selectively connected to a given bit line 204. A rowof the memory cells 208 may be memory cells 208 commonly connected to agiven word line 202. A row of memory cells 208 can, but need not,include all memory cells 208 commonly connected to a given word line202. Rows of memory cells 208 may often be divided into one or moregroups of physical pages of memory cells 208, and physical pages ofmemory cells 208 often include every other memory cell 208 commonlyconnected to a given word line 202. For example, memory cells 208commonly connected to word line 202 _(N) and selectively connected toeven bit lines 204 (e.g., bit lines 204 ₀, 204 ₂, 204 ₄, etc.) may beone physical page of memory cells 208 (e.g., even memory cells) whilememory cells 208 commonly connected to word line 202 _(N) andselectively connected to odd bit lines 204 (e.g., bit lines 204 ₁, 204₃, 204 ₅, etc.) may be another physical page of memory cells 208 (e.g.,odd memory cells). Although bit lines 204 ₃-204 ₅ are not explicitlydepicted in FIG. 2A, it is apparent from the figure that the bit lines204 of the array of memory cells 200A may be numbered consecutively frombit line 204 ₀ to bit line 204 _(M). Other groupings of memory cells 208commonly connected to a given word line 202 may also define a physicalpage of memory cells 208. For certain memory devices, all memory cellscommonly connected to a given word line might be deemed a physical pageof memory cells. The portion of a physical page of memory cells (which,in some embodiments, could still be the entire row) that is read duringa single read operation or programmed during a single programmingoperation (e.g., an upper or lower page of memory cells) might be deemeda logical page of memory cells. A block of memory cells may includethose memory cells that are configured to be erased together, such asall memory cells connected to word lines 202 ₀-202 _(N) (e.g., all NANDstrings 206 sharing common word lines 202). Unless expresslydistinguished, a reference to a page of memory cells herein refers tothe memory cells of a logical page of memory cells.

FIG. 2B is another schematic of a portion of an array of memory cells200B as could be used in a memory of the type described with referenceto FIG. 1, e.g., as a portion of array of memory cells 104. Likenumbered elements in FIG. 2B correspond to the description as providedwith respect to FIG. 2A. FIG. 2B provides additional detail of oneexample of a three-dimensional NAND memory array structure. Thethree-dimensional NAND memory array 200B may incorporate verticalstructures which may include semiconductor pillars where a portion of apillar may act as a channel region of the memory cells of NAND strings206. The NAND strings 206 may be each selectively connected to a bitline 204 ₀-204 _(M) by a select transistor 212 (e.g., that may be drainselect transistors, commonly referred to as select gate drain) and to acommon source 216 by a select transistor 210 (e.g., that may be sourceselect transistors, commonly referred to as select gate source).Multiple NAND strings 206 might be selectively connected to the same bitline 204. Subsets of NAND strings 206 can be connected to theirrespective bit lines 204 by biasing the select lines 215 ₀-215 _(L) toselectively activate particular select transistors 212 each between aNAND string 206 and a bit line 204. The select transistors 210 can beactivated by biasing the select line 214. Each word line 202 may beconnected to multiple rows of memory cells of the memory array 200B.Rows of memory cells that are commonly connected to each other by aparticular word line 202 may collectively be referred to as tiers.

FIG. 3 is a conceptual view of the organization of a die 354 containingan apparatus having an array of memory cells in accordance with anembodiment. The die 354 may contain one or more planes 352, with eachplane 352 containing a number of blocks of memory cells 350 of an arrayof memory cells. Each block of memory cells 350 may include memory cellsthat are capable of being erased concurrently. Blocks of memory cells350 of one plane 352 may be accessible (e.g., written to, read from, orerased) concurrently with blocks of memory cells 350 of a differentplane 352. In addition, a subset of the blocks of memory cells 350 of aplane 352 may be accessible concurrently with a different subset of theblocks of memory cells 350 of that plane 352.

FIG. 4 is a top view of a semiconductor wafer 356 containing a number ofdies 354 containing an apparatus having an array of memory cells inaccordance with an embodiment. The dies 354 are often fabricated on thesemiconductor wafer 356 concurrently, using the same processing to formthe same circuitry on each die 354. However, due to inherent variationsin industrial processing, different dies 354, and even differentportions of the same die 354, may demonstrate different performancecharacteristics. For example, different dies 354 of the semiconductorwafer 356, operating under the same parameters, may produce differingthreshold voltage distributions in response to an erase operations.Similarly, different blocks of memory cells of a single die 354,operating under the same parameters, may produce differing thresholdvoltage distributions in response to an erase operations. And memorycells coupled to different access lines of a single block of memorycells, operating under the same parameters, may produce differingthreshold voltage distributions in response to an erase operations.While differences are generally expected, some generalizations may bemade, either empirically or through testing, regarding how differentaccess lines, blocks of memory cells or dies may perform relative toeach other.

As previously discussed, memory cells are typically erased to removecharge from their data-storage structures prior to programming thememory cells to have a desired data state. Erase operations typicallyinvolve applying an elevated voltage, e.g., an erase pulse, to channelregions of the memory cells while applying a lower voltage to the accesslines coupled to those memory cells. An erase verify operation may thenbe performed to detect whether the memory cells are sufficiently erased,e.g., whether the memory cells have a distribution of threshold voltagesbelow some threshold, e.g., below some threshold voltage value deemed ahigh limit of erased threshold voltages. If the erase verify operationfails, an additional erase pulse might be applied.

Due to the processing variability and/or other factors, an erasethreshold voltage distribution for the memory cells coupled to oneaccess line of a block of memory cells may be different than the erasethreshold voltage distributions for the memory cells coupled to otheraccess lines of the block of memory cells. FIGS. 5A-5D depictvisualizations of erase threshold voltage distributions for individualaccess lines and resulting erase threshold voltage distributions.

FIG. 5A depicts a visualization of a plurality of erase thresholdvoltage distributions 501 as might be obtained in response to applyingan erase pulse while applying a particular voltage (e.g., 0V) to eachaccess line of a block of memory cells. Each erase threshold voltagedistribution 501 may correspond to the memory cells coupled to a singleaccess line of a block of memory cells. Alternatively, one or more ofthe erase threshold voltage distributions 501 may correspond to thememory cells coupled to a more than one access line of the block ofmemory cells. FIG. 5B depicts a visualization of a collective erasethreshold voltage distribution 502 for the block of memory cells, e.g.,representing a combination of the erase threshold voltage distributions501 from the most negative erase threshold voltage to the most positiveerase threshold voltage of the erase threshold voltage distributions501.

It may be desirable to reduce the width of the erase threshold voltagedistribution 502 of a block of memory cells. For example, programming ofmemory cells starting from a same initial threshold voltage, or at leasta more narrow distribution of threshold voltages, may be morepredictable and may facilitate more narrow distributions of thresholdvoltages for each resulting data state. Prior solutions include applyinga number of different voltages to the access lines of the block ofmemory cells receiving the erase pulse to de-bias some access lines toshift their resulting threshold voltage distributions.

FIG. 5C depicts a visualization of a plurality of erase thresholdvoltage distributions 501′ as might be obtained in response to applyingan erase pulse while applying a number of different voltages to theaccess lines of a block of memory cells. Each erase threshold voltagedistribution 501′ may correspond to the memory cells coupled to a singleaccess line of a block of memory cells. Alternatively, one or more ofthe erase threshold voltage distributions 501′ may correspond to thememory cells coupled to a more than one access line of the block ofmemory cells. In comparison to the example of FIG. 5A, the example ofFIG. 5C may produce a closer grouping of the erase threshold voltagedistributions 501′. As a result, the collective erase threshold voltagedistribution 502′ for the block of memory cells, e.g., representing acombination of the erase threshold voltage distributions 501′, depictedin FIG. 5D may be more narrow. However, multiple voltages requiremultiple voltage sources, e.g., internal voltage generation circuits,which can increase the size of the die containing the array of memorycells. This variability will be discussed with reference to a particularstructure of 3D NAND memory. However, various embodiments are suitablefor other array architectures.

FIG. 6A is a cross-sectional view of a portion of a block of memorycells as might be used with various embodiments. Three-dimensionalmemory arrays are typically fabricated by forming alternating layers ofconductors and dielectrics, forming holes in these layers, formingadditional materials on sidewalls of the holes to define gate stacks formemory cells and other gates, and subsequently filling the holes with asemiconductor material to define a pillar section to act as channels ofthe memory cells and gates. To improve conductivity of pillar sectionsand an adjacent semiconductor material, e.g., upon which they areformed, a conductive (e.g., conductively-doped) portion is typicallyformed in the pillar section at an interface with the adjacentsemiconductor material. These conductive portions are typically formedof a different conductivity type than the pillar section and adjacentsemiconductor material. For example, if the pillar section is formed ofa p-type semiconductor material, the conductive portion might have ann-type conductivity.

Forming holes through multiple layers typically produces holes ofdecreasing diameter toward the bottom of the holes due to the nature ofthe removal processes commonly used in the semiconductor industry. Tomitigate against the holes becoming too narrow, formation of arrays ofthe type described with reference to FIGS. 2A-2B, might be segmented,such that the layers for forming a first portion of the NAND string maybe formed, then portions may be removed to define holes, and theremaining structures may be formed within the holes. Following formationof the first portion of the NAND string, a second portion of the NANDstring might be formed over the first portion in a similar manner. FIG.6A depicts a structure of this type. In FIG. 6A, two strings ofseries-connected memory cells are depicted in the cross-sectional view.It is noted that the spaces between various elements of the figure mayrepresent dielectric material.

With reference to FIG. 6A, a first NAND string includes a first pillarsection 340 ₀₀ and a second pillar section 340 ₁₀. The first pillarsection 340 ₀₀ and the second pillar section 340 ₁₀ may each be formedof a semiconductor material of a first conductivity type, such as ap-type polysilicon. Conductive portions 342 ₀₀ and 342 ₁₀ may be formedat the bottoms of the pillar sections 340 ₀₀ and 340 ₁₀, respectively,with the conductive portion 342 ₀₀ electrically connected to the source216 and the conductive portion 342 ₁₀ electrically connected to thepillar section 340 ₀₀. The conductive portions 342 ₀₀ and 342 ₁₀ may beformed of a semiconductor material of a second conductivity typedifferent than the first conductivity type. For the example where thefirst pillar section 340 ₀₀ and the second pillar section 340 ₁₀ mayeach be formed of a p-type polysilicon, the conductive portions 342 ₀₀and 342 ₁₀ might be formed of an n-type semiconductor material, such asan n-type polysilicon. In addition, the conductive portions 342 ₀₀ and342 ₁₀ might have a higher conductivity level than the pillar sections340 ₀₀ and 340 ₁₀. For example, the conductive portions 342 ₀₀ and 342₁₀ might have an n+ conductivity. Alternatively, the conductive portions342 ₀₀ and 342 ₁₀ may be formed of a conductor, e.g., a metal or metalsilicide.

The pillar section 340 ₁₀ is electrically connected to the data line 204through a conductive plug 344 ₀. The conductive plug 344 ₀, in thisexample, might also be formed of a semiconductor material of the secondconductivity type, and may likewise have a higher conductivity levelthan the pillar sections 340 ₀₀ and 340 ₁₀. Alternatively, theconductive plug 344 ₀ may be formed of a conductor, e.g., a metal ormetal silicide. The first NAND string further includes a source selectgate at an intersection of the source select line 214 and the pillarsection 340 ₀₀, and a drain select gate at an intersection of the drainselect line 215 and the pillar section 340 ₁₀. The first NAND stringfurther includes a memory cell at an intersection of each of the accesslines 202 ₀-202 ₇ and the pillar sections 340 ₀₀ and 340 ₁₀. Thesememory cells further include data-storage structures 234 ₀₀-234 ₇₀.While the structure of FIG. 6A is depicted to include only eight accesslines 202 in an effort to improve readability of the figure, a typicalNAND structure might have significantly more access lines 202.

Although not all numbered, for clarity of FIG. 6A, data-storagestructures 234 are depicted on both sides of the pillar sections 340.Individual data-storage structures 234 may wrap completely around theirrespective pillar section 340, thus defining a data-storage structure234 for a single memory cell. Alternatively, structures are known havingsegmented data-storage structures 234, such that more than one (e.g.,two) memory cells are defined at each intersection of an access line 202and a pillar section 340. Embodiments described herein are independentof the number of memory cells defined around a pillar section 340.

To improve the conductivity across the conductive portion 342 ₁₀, thefirst NAND string may further include an intermediate gate at anintersection of the select line 217. This divides the memory cells ofthe first NAND string into a first deck of memory cells 240 ₀ and asecond deck of memory cells 240 ₁.

The decks of memory cells 240 can generally be thought of as groupingsof memory cells sharing a common pillar section 340, i.e., a singlepillar section 340 acting as channel regions for that grouping of memorycells, and can be extended to include a plurality of groupings of memorycells, where each such grouping of memory cells shares a common pillarsection 340, and the respective common pillar sections 340 are formed atthe same level (e.g., are intersected by the same access lines 202),which may include all such groupings of memory cells sharing a commonset (e.g., one or more) of access lines 202. For example, deck of memorycells 240 ₀ may include those memory cells formed at the intersectionsof access lines 202 ₀ and 202 ₁ with the pillar section 340 ₀₀. The deckof memory cells 240 ₀ may further include those memory cells formed atthe intersections of access lines 202 ₀ and 202 ₁ with their respectivepillar sections 340 ₀₀ and 340 ₀₁, and may still further include allmemory cells formed at the intersections of access lines 202 ₀ and 202 ₁with the pillar sections 340 ₀₀ and 340 ₀₁, and any other pillarsections 340 formed at the same level.

With further reference to FIG. 6A, a second NAND string includes thefirst pillar section 340 ₀₁ and a second pillar section 340 ₁₁. Thefirst pillar section 340 ₀₁ and a second pillar section 340 ₁₁ may eachbe formed of a semiconductor material of the first conductivity type,such as a p-type polysilicon. Conductive portions 342 ₀₁ and 342 ₁₁ maybe formed at the bottoms of the pillar sections 340 ₀₁ and 340 ₁₁,respectively, with the conductive portion 342 ₀₁ electrically connectedto the source 216 and the conductive portion 342 ₁₁ electricallyconnected to the pillar section 340 ₀₁. The conductive portions 342 ₀₁and 342 ₁₁ may be formed of a semiconductor material of the secondconductivity type. For the example where the first pillar section 340 ₀₁and a second pillar section 340 ₁₁ may each be formed of a p-typepolysilicon, the conductive portions 342 ₀₁ and 342 ₁₁ might be formedof an n-type semiconductor material, such as an n-type polysilicon. Inaddition, the conductive portions 342 ₀₁ and 342 ₁₁ might have a higherconductivity level than the pillar sections 340 ₀₁ and 340 ₁₁. Forexample, the conductive portions 342 ₀₁ and 342 ₁₁ might have an n+conductivity.

The pillar section 340 ₁₁ is electrically connected to the data line 204through a conductive plug 344 ₁. The conductive plug 344 ₁, in thisexample, might also be formed of a semiconductor material of the secondconductivity type, and may likewise have a higher conductivity levelthan the pillar sections 340 ₀₁ and 340 ₁₁. Alternatively, theconductive plug 344 ₁ may be formed of a conductor, e.g., a metal ormetal silicide. The second NAND string further includes a source selectgate at an intersection of the source select line 214 and the pillarsection 340 ₀₁, and a drain select gate at an intersection of the drainselect line 215 and the pillar section 340 ₁₁. The second NAND stringfurther includes a memory cell at an intersection of each of the accesslines 202 ₀-202 ₇ and the pillar sections 340 ₀₁ and 340 ₁₁. Thesememory cells further include data-storage structures 234 ₀₁-234 ₇₁.

To improve the conductivity across the conductive portion 342 ₁₁, thesecond NAND string further includes an intermediate gate at anintersection of the select line 217 and the pillar section 340 ₁₁. Thisdivides the memory cells of the second NAND string into the first deckof memory cells 240 ₀ and the second deck of memory cells 240 ₁.

FIG. 6B is a simplified cross-sectional view of a portion of a block ofmemory cells as might be used with various embodiments. In a structureof the type depicted in FIGS. 6A and 6B, memory cells coupled to accesslines near the middle of a deck 240 may tend to produce similardistributions of erase threshold voltages. However, differences may beexperienced for access lines at the top and/or bottom of the NAND string206, as well as near the interface between decks 240. One example ofde-biasing access lines for an erase operation on a block of thestructure depicted in FIG. 6B might include applying, during an erasepulse, 0V to the access lines 202 ₀ (e.g., WL₀) and 202 _(N) (e.g.,WL_(N)), 1V to the access lines 202 ₁ (e.g., WL₁) and 202 _(N−1) (e.g.,WL_(N−1)), and 2V to the access lines 202 _(x) (e.g., WL_(x)) through202 _(x+3) (e.g., WL_(x+3)). This would generally require threedifferent voltage supplies for the three different access line voltagesjust for the examples noted. Various embodiments seek to address thisvariation in erase threshold voltage distributions, and may facilitate afurther narrowing of the threshold voltage distribution of a block ofmemory cells with an accompanying reduction of a number of requiredvoltage supplies over the prior art.

FIG. 7 depicts a visualization of erase threshold voltage distributionsfor individual access lines in accordance with an embodiment. Forsimplification of the figure and discussion, only a small sampling oferase threshold voltage distributions 501 are shown. The erase thresholdvoltage distributions 501 may each represent the distribution of erasethreshold voltages of memory cells (e.g., of a block of memory cells)coupled to a single access line of a block of memory cells, or memorycells coupled to multiple access lines of the block of memory cells. Thememory cells corresponding to a particular erase threshold voltagedistribution 501 may include all memory cells couple to the access line(or access lines), or to some subset of those memory cells. The erasethreshold voltage distributions 501 may be the result of applying aparticular voltage to each of the access lines while applying an erasepulse to the channel regions of the corresponding memory cells. Inaddition, the erase threshold voltage distributions 501 might bedetermined, in whole or in part, empirically or through simulation.

In FIG. 7, the references 760 each represent a voltage differencebetween a target (e.g., desired) erase threshold voltage (Vt_(tar)) anda representative value of a corresponding erase threshold voltagedistribution 501. The representative value might be a mode, median ormean of the corresponding erase threshold voltage distribution 501. Itmight also be some other value representative of the corresponding erasethreshold voltage distribution 501, e.g., a minimum threshold voltage, amaximum threshold voltage, or some other value deemed representative.Determining the values of the erase threshold voltage distributions 501is known in the art, and may include applying a ramped read voltageduring an erase verify operation, and monitoring the number of memorycells activating at different levels of the ramped read voltage.However, the embodiments are not dependent on a particular method ofdetermining values of these distributions.

As can be seen in the example of FIG. 7, the representative value of theerase threshold voltage distribution 501 ₁ is lower than (e.g., lessthan) the target erase threshold voltage Vt_(tar) by the voltagedifference 760 ₁. The representative value of the erase thresholdvoltage distribution 501 ₀ is lower than the target erase thresholdvoltage Vt_(tar) by the voltage difference 760 ₀, which is greater thanthe voltage difference 760 ₁. In contrast, the representative value ofthe erase threshold voltage distributions 501 ₂ and 501 ₃ are eachhigher than (e.g., greater than) the target erase threshold voltageVt_(tar) by the voltage differences 760 ₂ and 760 ₃, respectively, withthe voltage difference 760 ₂ being less than the voltage difference 760₃.

From FIG. 7, it may be seen that if a second voltage, higher than theparticular voltage, was applied to the access line(s) corresponding tothe erase threshold voltage distribution 501 ₁ during the same erasepulse, its erase threshold voltage distribution 501 ₁ might shift towardthe target erase threshold voltage Vt_(tar), which might facilitatemoving its representative value to be similar to (e.g., equal to) thetarget erase threshold voltage Vt_(tar). In particular, by applying alower voltage differential between the channel region and the controlgate through the use of a higher control gate voltage, fewer chargesmight be expected to be removed from the data-storage structure, thusshifting the resulting erase threshold voltage distribution to theright. Similarly, if a third voltage, higher than the second voltage,was applied to the access line(s) corresponding to the erase thresholdvoltage distribution 501 ₀ during the same erase pulse, its erasethreshold voltage distribution 501 ₀ might shift toward the target erasethreshold voltage Vt_(tar) by a larger amount, which might facilitatemoving its representative value to be similar to (e.g., equal to) thetarget erase threshold voltage Vt_(tar).

Furthermore, if a fourth voltage, lower than the particular voltage, wasapplied to the access line(s) corresponding to the erase thresholdvoltage distribution 501 ₂ during the same erase pulse, its erasethreshold voltage distribution 501 ₂ might shift toward the target erasethreshold voltage Vt_(tar), which might facilitate moving itsrepresentative value to be similar to (e.g., equal to) the target erasethreshold voltage Vt_(tar). In particular, by applying a higher voltagedifferential between the channel region and the control gate through theuse of a lower control gate voltage, additional charges might beexpected to be removed from the data-storage structure, thus shiftingthe resulting erase threshold voltage distribution to the left.Similarly, if a fifth voltage, lower than the fourth voltage, wasapplied to the access line(s) corresponding to the erase thresholdvoltage distribution 501 ₃ during the same erase pulse, its erasethreshold voltage distribution 501 ₃ might shift toward the target erasethreshold voltage Vt_(tar) by a larger amount, which might facilitatemoving its representative value to be similar to (e.g., equal to) thetarget erase threshold voltage Vt_(tar).

For some embodiments, the target erase threshold voltage may bedetermined after determining the erase threshold voltage distributions501. For example, the target erase threshold voltage Vt_(tar) may bechosen such that no erase threshold voltage distribution 501 has arepresentative value greater than the target erase threshold voltageVt_(tar). Alternatively, the target erase threshold voltage Vt_(tar) maybe chosen such that no erase threshold voltage distribution 501 would beexpected to have a representative value greater than the target erasethreshold voltage Vt_(tar). In this manner, each erase threshold voltagedistribution 501 might be shifted toward the target erase thresholdvoltage Vt_(tar) by applying a voltage higher than the particularvoltage during a same erase pulse.

With regard to the example of FIG. 7, the differences between theparticular voltage and the second, third, fourth and fifth voltagesmight be referred to a bias differences. For some embodiments, the biasdifferences corresponding to the individual erase threshold voltagedistributions 501 might be deemed to be equal to their respectivevoltage differences 760, e.g., equal to (Vt_(tar)—the representativevalue). For other embodiments, a correction might be applied to therespective voltage differences 760. For example, each voltage difference760 might be multiplied by some constant corresponding to the respectiveaccess line(s). This constant might be the same for all access lines ofa block of memory cells, or there might be different constants fordifferent access lines. Values of such constants or other correctionsmight be determined experimentally, empirically or through simulation.

FIG. 8 is a flowchart of a method of operating a memory in accordancewith an embodiment. At 801, an erase threshold voltage distribution isdetermined for each access line of a plurality of access lines receivinga particular access line voltage during an erase pulse. For example, theplurality of access lines may include the access lines, e.g., wordlines, of a string of series-connected memory cells. For someembodiments, the plurality of access lines may include less than allaccess lines of a string of series-connected memory cells. For example,a sampling of some portion of the access lines of a string ofseries-connected memory cells might be used.

The erase pulse may be a voltage level applied to channel regions of thememory cells of a string of series-connected memory cells, such as byapplying the voltage level to a data line and source connected to thestring of series-connected memory cells, for a period of time. Theparticular access line voltage may be a voltage level applied to each ofthe access lines of the plurality of access lines for a period of timeconcurrent with the erase pulse. As used herein, a first act and asecond act occur concurrently when the first act occurs simultaneouslywith the second act for at least a portion of a duration of the secondact. For example, for at least a portion of the time of applying theerase pulse, the particular access line voltage is being appliedsimultaneously to the access lines of the plurality of access lines. Asone example, an erase pulse of 25V and a particular access line voltageof 0V might be expected to remove charge from a data-storage structureof a non-volatile memory cells.

A respective bias difference is determined at 803 for each access lineof the plurality of access lines. The respective bias difference isdetermined responsive to a difference between a representative value ofeach access line's erase threshold voltage distribution and a targeterase threshold voltage. The respective bias difference might be a valueof an access line voltage that would be expected to move therepresentative value of that access line's erase threshold voltagedistribution toward the value of the target erase threshold voltage hadit been applied to that access line during the erase pulse. Therepresentative value might be a mode, median or mean of thecorresponding erase threshold voltage distribution. It might also besome other value representative of the corresponding erase thresholdvoltage distribution, e.g., a minimum threshold voltage, a maximumthreshold voltage, or some other value deemed representative.

Optionally, at 805, the respective bias difference of a particularaccess line of the plurality of access lines might be used to determinea respective bias difference for a different access line. As an example,it may be deemed beneficial to not determine a respective biasdifference for each access line through the application of an erasepulse. For example, as noted with respect to 801, such testing mayinclude a sampling of access lines of a string of series-connectedmemory cells. Alternatively or in addition, such testing may include asampling of blocks of memory cells of a die containing those blocks ofmemory cells, a sampling of dies per semiconductor wafer containingthose dies, a sampling of dies per lot of multiple semiconductor waferscontaining those dies, etc., or some combination thereof. As such, therespective bias difference determined for one access line might be usedto determine the respective bias difference for one or more other accesslines of a same string of series-connected memory cells, for one or moreother access lines of different blocks of memory cells, for one or moreother access lines of different dies containing blocks of memory cells,etc.

The determination might be made for other access lines expected to havesimilar operating characteristics as the particular access line. Forsome situations, the determination may be simply an assignment of therespective bias difference of the particular access line to the otheraccess line. Similarly, where a difference in operating characteristicsbetween the particular access line and a different access line might beexpected to result in a particular relationship between their respectivebias differences, as might be determined through experimentation,empirical evidence or simulation, the respective bias difference of theparticular access line might be corrected in response to the particularrelationship in order to determine the respective bias difference of theother access line.

The respective bias differences for each access line may be stored forsubsequent use. For example, the respective bias differences might bestored to a trim register 126 or an array of memory cells 104 of amemory 100. For some embodiments, permissible values of the respectivebias differences may be predetermined. As one example, there might beeight different values of a bias difference represented by a three bitregister, e.g., 000, 001, 010, 011, 100, 101, 110 and 111 couldrepresent bias differences of 0V, 0.25V, 0.5V, 0.75V, 1V, 1.5V, 2V and2.5V, respectively. In such scenarios, the determined bias differencemight be the predetermined value that is closest to the determined(e.g., calculated) value. For some embodiments, the stored biasdifference may represent a voltage level to be applied during an eraseoperation while memory cells coupled to the respective access line areintended for erasure.

FIG. 9 is a flowchart of a method of operating a memory in accordancewith an embodiment. For example, the method of FIG. 9 may represent anerase operation. At 911, a first voltage is applied to channel regionsof a plurality of memory cells, with each memory of the plurality ofmemory cells coupled to a respective access line of a plurality ofaccess lines. For example, the plurality of access lines may include theaccess lines, e.g., word lines, of a string of series-connected memorycells. Note that the plurality of access lines may further be coupled toother memory cells, and that the first voltage may be applied to thechannel regions of the other memory cells concurrently. For example, thememory cells of a number of strings of series-connected non-volatilememory cells, e.g., a block of memory cells, may receive the firstvoltage to their channel regions concurrently.

At 913, a second voltage, lower than the first voltage is applied toeach access line of the plurality of access lines other than a first setof access lines of the plurality of access lines. The first set ofaccess lines may include one or more access lines of the plurality ofaccess lines. The second voltage applied to access lines other than thefirst set of access lines may be configured to inhibit erasure of theircorresponding memory cells also receiving the first voltage to theirchannel regions, e.g., inhibit removal of charge from the data-storagenodes of those memory cells.

At 915, a third voltage, lower than the second voltage, is applied tothe first set of access lines of the plurality of access lines whileapplying the first voltage to the channel regions of the plurality ofmemory cells and while applying the second voltage to each access lineof the plurality of access lines other than the first set of accesslines. The third voltage applied to the access lines of the first set ofaccess lines may be configured to cause erasure of their correspondingmemory cells also receiving the first voltage to their channel regions,e.g., cause removal of charge from the data-storage nodes of thosememory cells. The third voltage may be determined from the respectivebias difference corresponding to the access lines of the first set ofaccess lines. For example, for one or more access lines having aparticular bias difference, the third voltage may be equal to theparticular bias difference where the access line voltage applied duringa determination of the particular bias difference was 0V. For othersituations, the particular bias difference could be added to the accessline voltage applied during a determination of the particular biasdifference, for example. Note that the particular bias difference mightbe indirectly determined through testing of one or more different accesslines, and the particular bias difference might be a corrected value.For some embodiments, the first set of access lines may be a set ofaccess lines containing more than one access line, where those accesslines share similar respective bias differences. For example, thoseaccess lines whose respective bias differences each fall within aparticular range of values may be deemed to share similar respectivebias differences. The particular bias difference for the first set ofaccess lines may represent a mode, median, mean, or other valuerepresentative of the similar bias differences of the first set ofaccess lines.

At 917, a desired voltage level of the third voltage may be determinedfor a subsequent (e.g., different) set of access lines of the pluralityof access lines. For example, after some period of time, the thirdvoltage may be removed from the first set of access lines. That periodof time may correspond to a time deemed appropriate to cause erasure ofthe memory cells coupled to the first set of access lines. The desiredvoltage level of the third voltage may be determined from (e.g., changedto) some value corresponding to a respective bias difference for thesubsequent set of access lines of the plurality of access lines, and maybe determined as discussed with respect to 915. Generation of variousvoltage levels is well understood in the art, and might include, forexample, the use of an adjustable voltage divider, or adjustablereference voltages applied to a voltage divider. However, embodimentsare not dependent upon a method of generating the different voltagelevels.

At 919, after determining the desired voltage level of the third voltagefor the subsequent set of access lines, the third voltage is applied tothe subsequent set of access lines while applying the first voltage tothe channel regions of the plurality of memory cells and while applyingthe second voltage to each access line of the plurality of access linesother than the subsequent set of access lines. The third voltage appliedto the access lines of the subsequent set of access lines may beconfigured to cause erasure of their corresponding memory cells alsoreceiving the first voltage to their channel regions, e.g., causeremoval of charge from the data-storage nodes of those memory cells.

At 921, a decision is made whether the application of a voltage otherthan the second voltage is to be applied to any additional access linesof the plurality of access lines, e.g., whether there is a desire toerase any additional memory cells. If yes, the process returns 917 tochange the voltage level of the third voltage, e.g., to correspond to anext set of access lines of the plurality of access lines. This processmight be repeated until each access line of the plurality of accesslines receives a third voltage, with the voltage level of the respectivethird voltage being determined in response to a respective biasdifference and configured to cause erasure of their corresponding memorycells. Each set of access lines of the plurality of access lines may bemutually exclusive.

For some embodiments, each set of access lines of the plurality ofaccess lines includes a single access line. The process described withreference to FIG. 9 might be performed in sequence, e.g., proceedingfrom a first access line (e.g., word line 202 ₀ of FIG. 2A) to a lastaccess line (e.g., word line 202 _(N) of FIG. 2A) of a string ofseries-connected memory cells (e.g., NAND string 206 ₀). The process mayfurther be performed concurrently on all strings of series-connectedmemory cells of a block of memory cells (e.g., NAND strings 206 ₀-206_(M)). Other orders might also be used, e.g., proceeding from a lastaccess line to a first access line; proceeding in order of respectivebias differences, either from low to high, or high to low; or an orderbased on any other criteria set by a user of the apparatus.

For some embodiments, each set of access lines of the plurality ofaccess lines includes one or more access lines. The process describedwith reference to FIG. 9 might be performed in sequence, e.g.,proceeding from a first set of access lines (e.g., word lines 202 ₀-202_(a)), to a second set of access lines (e.g., word line 202 _(a+1)-202_(b)), to a third set of access lines (e.g., word line 202 _(b+1)-202_(c)), and so one, of a string of series-connected memory cells (e.g.,NAND string 206 ₀). The process may further be performed concurrently onall strings of series-connected memory cells of a block of memory cells(e.g., NAND strings 206 ₀-206 _(M)). Other orders might also be used,e.g., proceeding from a last set of access lines to a first set ofaccess lines, or proceeding in order of respective bias differences,either from low to high, or high to low. The sets of access lines usedin one block of memory cells may be the same or different in otherblocks of memory cells for a given die containing those blocks of memorycells.

FIG. 10 depicts waveforms of access lines and channel regions for amethod of operating a memory in accordance with an embodiment. Thewaveforms of FIG. 10 might represent waveforms of the method ofoperating a memory as discussed with reference to FIG. 9. In FIG. 10,the waveform 1070 may represent a voltage (e.g., the first voltage ofFIG. 9) applied to channel regions of a plurality of memory cells, e.g.,the memory cells of one or more strings of series-connected non-volatilememory cells. As one example, the voltage level of the waveform 1070 maybe 25V. The waveform 1072 may represent a voltage (e.g., the secondvoltage of FIG. 9) applied to access lines for which there is a desireto inhibit erasure of their corresponding memory cells. As one example,the voltage level of the waveform 1072 may be 20V. The waveforms 1074,1076 and 1078 may represent the different voltage levels of the voltage(e.g., the third voltage of FIG. 9) applied to different sets of accesslines at different time periods in order to cause erasure of thecorresponding memory cells.

For example, at time to, the voltage of waveform 1074 might be appliedto a first set of access lines. The first set of access lines maycontain one or more access lines. The first set of access lines maycontain a contiguous grouping of one or more access lines.Alternatively, the first set of access lines may contain access linesthat are expected (e.g., through experimentation, empirical evidence orsimulation) to have similar operating characteristics, whethercontiguous or not. Access lines might be deemed to have similaroperating characteristics when their expected erase thresholddistributions under the same erase conditions have representative valueswithin a particular range of values. As one example, access lines havinga determined bias difference closest to a same value of a set ofpermissible values might be deemed to have similar operatingcharacteristics. At time t1, the voltage of waveform 1070 may be appliedto the channel regions of the memory cells while the voltage of thewaveform 1072 is applied to access lines other than the first set ofaccess lines. The voltage of waveform 1074 applied to the first set ofaccess lines might be allowed to attain its desired voltage level 1080prior to applying the voltage of the waveform 1070 to the channelregions of the memory cells, e.g., to avoid applying a voltagedifference between the channel region and the control gate of a memorycell that is larger than desired in view of the respective biasdifference of its access line.

At time t2, the voltage of the waveforms 1070 and 1072 may attain theirdesired voltage levels. For a time period from time t2 to time t3, thefirst set of access lines receives the voltage level 1080 whileremaining access lines receive the voltage of the waveform 1072 andwhile the channel regions of the memory cells receive the voltage of thewaveform 1070. The time period from time t2 to time t3 may be a timeperiod expected to be sufficient to remove charge to a particular level,e.g., such that memory cells coupled to the first set of access linesmight be expected to have an erase threshold voltage distribution havinga representative value equal to the target erase threshold voltage. Thevoltage level 1080 may be determined from (e.g., calculated from orequal to), the respective bias difference of the first set of accesslines. At time t3, the voltage applied to the first set of access linesis transitioned (e.g., switched) to the voltage of the wave form 1072.

Also at time t3, the voltage applied to a subsequent set of access linesmay be transitioned (e.g., switched) from the voltage of the waveform1072 to the voltage of the waveform 1076. The subsequent set of accesslines may contain one or more access lines. The subsequent set of accesslines may contain a contiguous grouping of one or more access lines, andthe contiguous grouping of the subsequent set of access lines may or maynot be immediately adjacent to a contiguous grouping of the first set ofaccess lines. Alternatively, the subsequent set of access lines maycontain access lines that are expected (e.g., through experimentation,empirical evidence or simulation) to have similar operatingcharacteristics, whether contiguous or not. At time t4, the voltage ofthe waveform 1076 may attain its desired voltage level 1082. For a timeperiod from time t4 to time t5, the subsequent set of access linesreceives the voltage level 1082 while remaining access lines receive thevoltage of the waveform 1072 and while the channel regions of the memorycells receive the voltage of the waveform 1070. The time period fromtime t4 to time t5 may be a time period expected to be sufficient toremove charge to a particular level, e.g., such that memory cellscoupled to the subsequent set of access lines might be expected to havean erase threshold voltage distribution having a representative valueequal to the target erase threshold voltage. The time period from timet4 to t5 may be equal to the time period from time t2 to time t3. Thevoltage level 1082 may be determined from (e.g., calculated from orequal to), the respective bias difference of the subsequent set ofaccess lines. At time t5, the voltage applied to the subsequent set ofaccess lines is transitioned (e.g., switched) to the voltage of thewaveform 1072.

Also at time t5, the voltage applied to a subsequent (e.g., nextsubsequent) set of access lines may be transitioned (e.g., switched)from the voltage of the waveform 1072 to the voltage of the waveform1078. The next subsequent set of access lines may contain one or moreaccess lines. The next subsequent set of access lines may contain acontiguous grouping of one or more access lines, and the contiguousgrouping of the next subsequent set of access lines may be immediatelyadjacent to a contiguous grouping of the prior subsequent set of accesslines. Alternatively, the next subsequent set of access lines maycontain access lines that are expected (e.g., through experimentation,empirical evidence or simulation) to have similar operatingcharacteristics, whether contiguous or not. At time t6, the voltage ofthe waveform 1078 may attain its desired voltage level 1084. For a timeperiod from time t6 to time t7, the next subsequent set of access linesreceives the voltage level 1084 while remaining access lines receive thevoltage of the waveform 1072 and while the channel regions of the memorycells receive the voltage of the waveform 1070. The time period fromtime t6 to time t7 may be a time period expected to be sufficient toremove charge to a particular level, e.g., such that memory cellscoupled to the next subsequent set of access lines might be expected tohave an erase threshold voltage distribution having a representativevalue equal to the target erase threshold voltage. The time period fromtime t6 to time t7 may be equal to the time period from time t2 to timet3. The voltage level 1084 may be determined from (e.g., calculated fromor equal to), the respective bias difference of the next subsequent setof access lines. At time t7, the voltage applied to the subsequent setof access lines is transitioned (e.g., switched) to the voltage of thewaveform 1072 and the process might be repeated for additionalsubsequent sets of access lines, e.g., until all of the access lineshave received a voltage level corresponding to their respective biasdifference while their memory cells have received the voltage of thewaveform 1070 applied to their channel regions.

CONCLUSION

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement that is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. Many adaptations ofthe embodiments will be apparent to those of ordinary skill in the art.Accordingly, this application is intended to cover any adaptations orvariations of the embodiments.

1. A method, comprising: applying a first voltage to channel regions of a plurality of memory cells, with each memory cell of the plurality of memory cells coupled to a respective access line of a plurality of access lines; applying a second voltage, having a voltage level lower than a voltage level of the first voltage, to each access line of the plurality of access lines other than a first set of access lines of the plurality of access lines; applying a third voltage, having a voltage level lower than the voltage level of the second voltage, to the first set of access lines while applying the first voltage to the channel regions of the plurality of memory cells and while applying the second voltage to each access line of the plurality of access lines other than the first set of access lines; determining a desired voltage level of the third voltage for a subsequent set of access lines of the plurality of access lines; and after determining the desired voltage level of the third voltage for the subsequent set of access lines, applying the third voltage to the subsequent set of access lines while continuing to apply the first voltage to the channel regions of the plurality of memory cells and while applying the second voltage to each access line of the plurality of access lines other than the subsequent set of access lines.
 2. The method of claim 1, wherein the subsequent set of access lines is a first subsequent set of access lines, the method further comprising: determining a desired voltage level of the third voltage for a next subsequent set of access lines of the plurality of access lines; and after determining the desired voltage level of the third voltage for the next subsequent set of access lines, applying the third voltage to the next subsequent set of access lines while continuing to apply the first voltage to the channel regions of the plurality of memory cells and while applying the second voltage to each access line of the plurality of access lines other than the next subsequent set of access lines.
 3. The method of claim 1, wherein the first set of access lines and the subsequent set of access lines are mutually exclusive sets of access lines.
 4. The method of claim 3, wherein the first set of access lines consists of a single access line of the plurality of access lines.
 5. The method of claim 4, wherein the subsequent set of access lines consists of a single access line of the plurality of access lines.
 6. The method of claim 3, wherein the first set of access lines comprises a contiguous grouping of one or more access lines coupled to respective memory cells of a string of series-connected memory cells of the plurality of memory cells.
 7. The method of claim 6, wherein the subsequent set of access lines comprises a contiguous grouping of one or more access lines coupled to respective memory cells of the string of series-connected memory cells.
 8. The method of claim 7, wherein the contiguous grouping of the subsequent set of access lines is contiguous with the contiguous grouping of the first set of access lines.
 9. The method of claim 3, wherein the first set of access lines comprises a set of access lines deemed to have similar operating characteristics.
 10. The method of claim 9, wherein the set of access lines deemed to have similar operating characteristics is not contiguous.
 11. The method of claim 1, the method further comprising: for each subsequent set of access lines of the plurality of access lines for a plurality of subsequent sets of access lines of the plurality of access lines: determining a desired voltage level of the third voltage for that subsequent set of access lines of the plurality of access lines; and after determining the desired voltage level of the third voltage for that subsequent set of access lines, applying the third voltage to that subsequent set of access lines while continuing to apply the first voltage to the channel regions of the plurality of memory cells and while applying the second voltage to each access line of the plurality of access lines other than that subsequent set of access lines.
 12. The method of claim 1, further comprising: determining an erase threshold voltage distribution for each access line of first set of access lines in response receiving a particular access line voltage during an erase pulse; determining a respective bias difference of the first set of access lines responsive to a difference between a representative value of the erase threshold voltage distribution and a target erase threshold voltage; and determining a voltage level of the third voltage for the first set of access lines from the respective bias difference of the first set of access lines.
 13. The method of claim 12, further comprising: determining an erase threshold voltage distribution for each access line of subsequent set of access lines in response receiving the particular access line voltage during the erase pulse; determining a respective bias difference of the subsequent set of access lines responsive to a difference between a representative value of its erase threshold voltage distribution and the target erase threshold voltage; and determining the desired voltage level of the third voltage for the subsequent set of access lines from the respective bias difference of the subsequent set of access lines.
 14. A method, comprising: determining an erase threshold voltage distribution for each access line of a plurality of access lines receiving a particular access line voltage during an erase pulse; for each access line of the plurality of access lines, determining a respective bias difference responsive to a difference between a representative value of its erase threshold voltage distribution and a target erase threshold voltage.
 15. The method of claim 14, wherein the erase pulse comprises a applying a voltage to channel regions of memory cells coupled to the access lines of the plurality of access lines.
 16. The method of claim 14, wherein a difference between the voltage applied to the channel regions of the memory cells coupled to the access lines of the plurality of access lines and the particular access line voltage is configured to cause erasure of data-storage nodes of the memory cells coupled to the access lines of the plurality of access lines.
 17. The method of claim 14, further comprising: for a particular access line of the plurality of access lines, using its respective bias difference to determine a respective bias difference for a different access line.
 18. The method of claim 14, further comprising selecting the target erase threshold voltage such that the difference between the target erase threshold voltage and the representative value of the respective erase threshold voltage distribution for a given access line of the plurality of access lines is expected to be a value greater than or equal to zero for each access line of the plurality of access lines.
 19. A method, comprising: applying a first voltage to channel regions of a plurality of memory cells, with each memory cell of the plurality of memory cells coupled to a respective access line of a plurality of access lines; for each set of access lines of a plurality of sets of access lines of the plurality of access lines: applying a second voltage, having a voltage level lower than a voltage level of the first voltage, to each access line of the plurality of access lines other than that set of access lines; determining a desired voltage level of a third voltage, having a voltage level lower than the voltage level of the second voltage, for that set of access lines; applying the third voltage for that set of access lines to that set of access lines while continuing to apply the first voltage to the channel regions of the plurality of memory cells and while applying the second voltage to each access line of the plurality of access lines other than that set of access lines; after a time period corresponding to that set of access lines, transitioning that set of access lines to the second voltage.
 20. The method of claim 19, wherein determining the desired voltage level of the third voltage for a given set of access lines of the plurality of sets of access lines comprises reading a stored value corresponding to that desired voltage level.
 21. The method of claim 19, wherein applying the first voltage to the channel regions of the plurality of memory cells while applying the second voltage to a given access line is configured to inhibit erasure of memory cells coupled to that given access line.
 22. The method of claim 21, wherein applying the first voltage to the channel regions of the plurality of memory cells while applying the third voltage to a given set of access lines is configured to cause erasure of memory cells coupled to that given set of access lines.
 23. The method of claim 19, wherein determining the desired voltage level of the third voltage for each set of access lines of the plurality of sets of access lines comprises determining a voltage level expected to cause a respective erase threshold voltage distribution for each set of access lines of the plurality of sets of access lines to have respective representative values approaching a target erase threshold voltage.
 24. The method of claim 19, wherein the time periods corresponding to each respective set of access lines of the plurality of sets of access lines are each a same length of time.
 25. The method of claim 19, wherein a first set of access lines of the plurality of sets of access lines to receive its third voltage and a next set of access lines of the plurality of sets of access line to receive its third voltage do not form a contiguous set of access lines of the plurality of access lines.
 26. The method of claim 25, wherein access lines of the first set of access lines are not contiguous with each other. 